In many process engineering installations, liquids are distributed in a gas. In such cases, it is often of decisive importance that the liquid is sprayed in drops that are as fine as possible. The finer the drops, the greater the specific surface area of the drops. This can give rise to considerable process engineering advantages. For example, the size of a reaction vessel and its production costs depend considerably on the average drop size. However, it is often by no means adequate for the average drop size to be below a certain limit value. Even a few significantly larger drops can lead to considerable operational malfunctions. This is the case in particular whenever the drops do not evaporate quickly enough on account of their size, so that drops or even pasty particles are deposited in downstream components, for example on filter fabrichoses or fan blades, and lead to operational malfunctions due to encrustations or corrosion.
In order to spray liquids finely, either high-pressure one-substance nozzles or medium-pressure two-substance nozzles are used. An advantage of two-substance nozzles is that they have relatively large flow cross sections, so that even liquids containing coarse particles can be sprayed.
The representation of FIG. 1 shows a two-substance nozzle with internal mixing according to the prior art. A basic problem with such nozzles results from the fact that the walls of the mixing chamber 7 are wetted with liquid. The liquid which wets the wall in the mixing chamber 7 is driven to the nozzle mouth as a liquid film 20 by the shearing stress and compressive forces. It is tempting to assume that the walls toward the nozzle mouth are blasted dry because of the high flow velocity of the gas phase, and that only very fine drops are thereby formed from the liquid film. However, theoretical and experimental work by one of the inventors, have shown that liquid films on walls may still exist as stable films without drop formation even when the gas flow that drives the liquid films to the nozzle mouth reaches supersonic speed. And this is indeed also the reason why it is possible to use liquid film cooling in rocket thrust nozzles.
The liquid films 20 that are driven by the gas flow to the nozzle mouth 8 may even migrate around a sharp edge at the nozzle mouth on account of the adhesive forces. They form a bead of water 12 on the outside of the nozzle mouth 8. Outer drops 13, the diameter of which is many times the average diameter of the drops in the jet core or the core jet 21, break away from this bead of water 12. And although these large outer drops only contribute to a small proportion of the mass, they are ultimately determinative for the dimensions of a vessel in which, for example, the temperature of a gas is to be lowered by evaporative cooling from 350° C. to 120° C. without drops entering a downstream fan or downstream fabric filter.
A liquid is introduced into the prior-art nozzle represented in FIG. 1 in the direction of the arrow 1, parallel to a center longitudinal axis 24. The liquid is passed through a lance tube 2, extending concentrically with respect to the center longitudinal axis 24, and enters a mixing chamber 7 at a liquid inlet 10. The lance tube 2 and the mixing chamber 7 are concentrically surrounded by an annular chamber 6, which is formed by means of a further lance tube 4 for the feeding of the compressed gas to the two-substance nozzle. Compressed gas is introduced into this annular chamber 6 according to the arrow 15. A circumferential wall of the mixing chamber 7 that is radial with respect to the center longitudinal axis 24 has a number of compressed gas inlets 5, which are arranged radially with respect to the center longitudinal axis 24. Through these compressed gas inlets 5, compressed gas can enter the mixing chamber 7 at right angles to the liquid jet entering through the liquid inlet 10, so that a liquid/air mixture is formed in the mixing chamber 7. The mixing chamber 7 is adjoined by a frustoconical constriction 3, which forms a convergent outlet portion, which is followed after an extremely narrow cross section 14 in turn by a frustoconical widening 9, which forms a devergent outlet portion. The frustoconical widening 9 ends at the outlet opening or the nozzle mouth 8.